The subject matter disclosed herein relates to a method and system for boosting the magnitude of the voltage on the direct current (DC) link of an inverter. More specifically, one inverter section operating in parallel with another inverter section may be selectively configured to either boost the magnitude of the voltage on the DC bus or convert the DC power to alternating current (AC) power.
There is an ever increasing demand for energy supplied by renewable energy sources. The power supplied by many renewable energy sources varies in magnitude and/or frequency. For example, photovoltaic (PV) arrays typically generate DC power which is dependent on the amount of light reaching the array and wind turbines typically generate AC power that varies in frequency according to the velocity of the turbine blades driving the generators. However, electronic devices are designed to be operated by power supplied at a fixed magnitude and frequency, such as the utility grid. Consequently, power converters are utilized with these power sources to convert the variable power supplied by the renewable energy source into power supplied at a fixed magnitude and frequency.
Power converters are available in many configurations. For example, power converters may convert DC to AC, AC to DC, DC at a first voltage level to DC at a second voltage level, or AC at a first frequency to AC at a second frequency. PV arrays typically generate DC power and utilize a power converter to convert the power generated by the array into AC power compatible with a utility grid. Wind turbines typically drive AC generators and utilize a two-stage converter that first converts the variable frequency AC power into DC power and subsequently converts this DC power into AC power compatible with the utility grid. Pulse width modulation (PWM) is one well known technique for converting DC power into AC power compatible with the utility grid.
Pulse width modulation is a high speed switching technique used to convert a DC voltage to an AC output voltage having a desired magnitude and frequency. Over a predefined switching period, the DC voltage is connected to the output for a percentage of the switching period, such that the output voltage alternates between zero volts and the DC voltage. The resulting average DC voltage observed at the output is equal to the magnitude of the switched DC voltage multiplied by the percentage of the switching period during which the reference voltage is connected to the output. The PWM routine may vary this percentage during each switching period such that the average DC voltage at the output changes at a desired AC output frequency. If the switching frequency is much greater than the desired AC output frequency, the voltage observed at the output approximates an AC output voltage.
Because the AC output voltage is generated by PWM switching of the DC reference voltage, the maximum value that the peak AC output voltage may be is equal to the magnitude of the switched DC voltage. Consequently, the DC voltage present in the power converter must be equal to or greater than the peak value of the desired AC output voltage in order for the converter to generate the desired output voltage.
However, due to the variable nature of many renewable energy sources, the DC voltage generated by the energy source is not always greater than the peak value of the desired AC output voltage. For example, the amount of power generated by PV arrays typically follows a bell-shaped curve. During early morning or late evening hours, the amount of power generated by the PV array drops below a minimum power threshold at which the necessary DC voltage to the converter may be maintained. Similarly, wind turbines have a cut-in speed, which is the minimum wind speed at which the wind turbine may operate. If the wind speed drops below this cut-in speed, the amount of power generated by the turbine is again not sufficient to maintain the necessary DC voltage for the converter.
Historically, attempts to overcome this limitation include lowering the magnitude of the desired output voltage. For example, a transformer may be included at the output of the converter. Utilizing the transformer to step-up the output voltage from a lower magnitude to match the magnitude of the grid voltage permits a lower desired output voltage from the power converter and, therefore, the power converter may continue operating at lower DC voltage levels.
However, including a transformer on the output of the converter is not without drawbacks. First, the transformer itself may require a substantial increase in the cost of the system. Second, the renewable energy source generates a specific amount of power, which is the product of the output voltage multiplied by the output current. If the power level remains the same and the output voltage is reduced, the output current necessarily increases. Consequently, the power converter components must be sized to handle the increased current, which similarly increases the cost of the system.
Other attempts to overcome the limitation of a low DC voltage include adding a DC boost converter to the input of the power converter. The DC boost converter increases the DC voltage on the DC link if the voltage level is not of sufficient magnitude to convert the DC voltage to an AC voltage of the desired magnitude. However, adding the DC boost converter requires additional switching devices and their related control components, again increasing the cost of the system.
Thus, it would be desirable to provide a power converter capable of operating over an increased range of DC voltages without a significant increase in cost.